These days, the protection of possessions and persons is a continuously growing phenomenon and the proof of this is the sales of alarm systems which have considerably increased in recent years, in particular driven by insurance companies.
Alarm systems include car alarms, anti-intrusion alarms to protect against intrusion within premises, video surveillance systems, etc.
It should be noted that the use of video surveillance systems in a home environment is far from being popularized, in particular due to the cost associated with such systems.
Indeed, a video surveillance system requires the purchase of surveillance cameras, storage and display units for the videos taken by the cameras and the setting up of a local communication network dedicated to the transfer of the video data.
It would consequently be desirable to have a video surveillance system able to use a local home network not dedicated to a particular application, such as video surveillance, to which may be connected a great variety of multimedia equipment sending or receiving data.
In a video surveillance network, several surveillance cameras take images simultaneously and encode them in a compressed manner.
A user of a central computer connected to the different surveillance cameras via a communication network will receive those compressed videos, will store them and view them on his computer.
However, it will not be possible for the computer to simultaneously view a high number of video sequences, both due to the difficulty for a human being to simultaneously view several video sequences with sustained attention, and due to the fact that the calculation capacities of the central computer, in particular in a local home network, generally do not enable a high number of video sequences to be decoded in real time.
Furthermore, it is very probable that the local home network will not have a bandwidth compatible with the simultaneous transport of numerous videos at full quality.
Hence the user of such a video surveillance system will prefer to view only relevant video sequences, in which movement occurs.
Furthermore, it would be more comfortable for the user to be able to have a video surveillance system able to automatically select the relevant video sequences without the user having to intervene.
From the international application WO 01 62005 entitled “Remote surveillance system” a video surveillance system is known in which the user must make an explicit request in order to obtain the video data captured by a surveillance camera.
Once the video data have been received from a video camera, the user may adjust the quality of those video data by modifying the frame rate, the format, and by filtering the different images constituting that video.
The adjustment of quality is made via a graphical interface and requires intervention by the user.
From the patent U.S. Pat. No. 5,751,346 a video surveillance system is also known which is used in the context of the surveillance of automatic teller machines and which automatically adjusts the frame rate of the images of the video sequence by analyzing the changes made between two images.
This analysis is made at each camera.
Such a system does not make it possible to conjointly adjust the quality of all the videos captured by the different cameras of a video surveillance system.
It should moreover be noted that the object of that video surveillance system is not to directly transmit video sequences, but rather to store them.
A similar system is also known from the patent U.S. Pat. No. 6,480,225.
In these last two patents, the simultaneous detection of alarms by several cameras is not taken into account, in that they detect a significant movement at the same time.
In another patent, U.S. Pat. No. 6,411,209, a video surveillance system uses a continuously operating camera coupled to an anti-intrusion alarm system such as an infrared sensor.
When an alarm is detected, the camera having detected the alarm stores a portion of video corresponding to a period encompassing the event which triggered the alarm. An analysis which is based on movement detection methods, recognition of forms or faces is then made at the camera.
If the alarm is confirmed further to the aforementioned analysis, the most relevant images of the portion of video sequence analyzed are transmitted to a central computer where a user operates.
Such a system does not make it possible to adjust the spatial quality of the video sequence, nor to make overall adjustment of the quality of each video sequence captured at each of the different cameras of the network taking into account the constraints related to the central computer and the bandwidth available on the network.
Moreover, it is not explained how that system makes it possible to take into account the simultaneous detection of alarms by several cameras for which there is in fact a risk of mutual hindrance at the time of the simultaneous transmission of video sequences to the central computer.
Moreover, simultaneous detections of alarms risk causing congestion of the network when the data are later transmitted simultaneously or practically simultaneously.
Furthermore, that system requires a high calculation capacity at each camera.
A video surveillance system is also known from the patent US 6,456,321 using a camera which produces both a highly compressed signal of low quality and a signal with little compression.
When no alarm is detected by the camera, only the highly compressed video signal is transmitted over a network to the central computer where a user operates, whereas the other video signal with low compression is stored at the camera.
When an alarm is triggered, the highly compressed signal is displayed, after decompression, on the screen of the user who may then decide whether or not to receive the video signal with little compression present at the camera.
Here too, action by the user is necessary in dealing with the alarms.
The aforementioned video surveillance system does not make it possible to automatically adjust the quality of video sequences globally for the entire network.
That system does not make it possible to take into account the simultaneous detection of alarms by several cameras either.
In addition, from the document U.S. Pat. No. 6,323,897, a video surveillance system is known in which several cameras are connected to a central server via a communication network of TCP/IP type.
On each camera is included, in particular, a movement detector, a microphone and an analyzer of the traffic conveyed by the network.
Where the traffic is low, all the cameras can send video and audio data continuously to the server. However, when the traffic becomes high, only the camera which has detected an alarm will send.
Depending on how busy the network is, that camera will send audio and video data, solely audio data, or else solely data characterizing the alarm such as data concerning the position of the camera.
On reception of those alarm signals, the server will decide to increase the priority of the data coming from the camera concerned in order to terminate the sending of data by the other cameras.
That system is not entirely satisfactory since it does not make it possible to take into account the simultaneous detection of alarms by two or more cameras.
This is because several cameras that have simultaneously detected an alarm risk mutual hindrance when transmitting video sequences to the server.
In addition, when the network is saturated, no mechanism is provided to remedy the lack of reception of the alarm signals by the server.